The present invention relates to a circuit arrangement for evaluating received oscillations whose maximum possible time duration is predetermined, by determining the time duration of the oscillations by means of a counter which receives pulses during the occurrence of the oscillation and by storing the counted pulses as information. More particularly the present invention relates to such a circuit arrangement for ultrasonic remote control of a television receiver.
For remote control of an instrument there exists the requirement of substantially eliminating interfering influences caused by noise or permanent interference sources which have a frequency identical to the useful frequency of the transmitted signal. In an ultrasonic remote control device for television receivers described in the periodical "Funkschau" 1973, Issue No. 18, pages 675-677, a desired operating function is produced in that two ultrasonic signals of different frequencies are transmitted in succession by an ultrasonic signal generator and are received by an ultrasonic signal receiver. The two immediately successive frequencies are coded in their value and in their respective duration. The frequency of the first ultrasonic signal determines whether a channel is to be selected or a level is to be changed, while the duration of this signal component determines the number of the channel or the type and direction of change in level, respectively. The actuation of the performance of the previously stored instruction takes place only after receipt of the second ultrasonic signal at the other frequency. Thus the above-mentioned requirement is met in part in this known ultrasonic remote control device. If an interference signal appears which has a frequency equal to that of the first ultrasonic signal, this signal is received in the same manner as a useful signal and the corresponding information is stored, but an error in control can occur only if following the above-mentioned interference signal a second interference signal with the frequency of the second ultrasonic signal appears which actuates the performance of the previously stored instruction.